Birds, especially those who eat crops, have been brought troubles to farmers for centuries. The use of a scarecrow is a traditional way to prevent birds from approaching crops. With the development in science and technology, the role of a scarecrow is replaced by an electronic bird-expelling device such as the one disclosed in Huang, Taiwanese Patent Publication No. 308797 filed Nov. 9, 1996, which is incorporated herein for reference. Huang's bird-expelling device has to be manually controlled through a main frame to control the ON/OFF operations of sounding devices which are distributed in the farm and interconnected to the main frame through transmission wires. The sounding devices are stored therein audio data of eagle's sound, and sounding eagle's whistle to scare birds away while being turned ON. Accordingly, complicated transmission wires are needed for interconnecting the sounding devices with the main frame. Further, the continuous eagle sounding not only wastes energy but also results in noise pollution.